Wapple Architect Mobile Plugin for WordPress is a plugin that allows you to mobilize your blog in minutes.
Head over to Wapple and fill out the simple form, you should be able to get your dev key within a couple of minutes!
Have you entered your dev key into the Wapple Architect settings? If not, head over to Settings > Wapple Architect (or Options > Wapple Architect if you're on version 2.3.3 and under) and enter it into the "Dev Key" input box.
In older versions of the plugin, you needed SOAP and SimpleXML, but not any more! From version 1.5.13, the dependancy on SimpleXML has been totally removed! You also do not need SOAP and can communicate with Wapple's web services via REST.
If you want to upload a custom mobile header image, check out the settings area, it's in the section titled: "Basic Settings" and at the bottom there's a section called "Mobile Theme Settings" - your mobile stylesheet is in there!
You're free to customize any of these, infact, the more you customize the CSS, the better your site will look!
Check write permissions on the /wp-content/uploads/ folder, it needs to be writable!